We study asymptotic normality of the randomized periodogram estimator of quadratic variation in the mixed Brownian-fractional Brownian model. In the semimartingale case, that is, where the Hurst parameter H of the fractional part satisfies H ∈ (3/4, 1), the central limit theorem holds. In the nonsemimartingale case, that is, where H ∈ (1/2, 3/4], the convergence toward the normal distribution with a nonzero mean still holds if H = 3/4, whereas for the other values, that is, H ∈ (1/2, 3/4), the central convergence does not take place. We also provide Berry-Esseen estimates for the estimator.